1. Field of the Invention
The present invention is in the field of early warning devices for fire detection and more particularly relates to a compact apparatus which measures the concentration and the rate of change in the concentration of carbon dioxide at the onset of a fire as a means for its early and rapid detection.
2. The Prior Art
The fire detectors that are available commercially today fall into three basic classifications, namely flame sensing, thermal and smoke detectors. This classification is designed to respond to the three principal types of energy and matter characteristics of a fire environment: flame, heat and smoke.
The flame sensing detector is designed to respond to the optical radiant energy generated by the diffusion flame combustion process--the illumination intensity and the frequency of flame modulation. Two types of flame detectors are commonly in use: the ultraviolet (UV) detectors which operate beyond the visible at wavelengths below 4,000A, and the infrared detectors which operate in the wavelengths above 7,000 A. To prevent false signals from the many sources of ultraviolet and infrared optical radiation present in most hazard areas the detectors are programmed to respond only to radiation with frequency modulation within the flicker frequency range for flame (5-30 Hz).
Flame detectors generally work well and seldom generate false alarms However, they are relatively complex and expensive fire detectors which are not amenable to low-cost and mass-oriented usage. Instead they are mostly utilized in specialized high-value and unique protection areas such as aircraft flight simulators, aircraft hangars, nuclear reactor control rooms, etc.
Thermal detectors are designed to operate from thermal energy output--the heat--of a fire. This heat is dissipated throughout the area by laminar and turbulent convection flow. The latter is induced and regulated by the fire plume thermal column effect of rising heated air and gases above the fire surface. There are two basic types of thermal detectors: the fixed temperature type and the rate of rise detector type. The fixed temperature type further divides into the spot type and the line type. The spot detector involves a relatively small fixed unit with a heat-responsive element contained within the unit or spot location of the detector. With the line detector the thermal reactive element is located along a line consisting of thermal-sensitive wiring or tubing. Line detectors can cover a greater portion of the hazard area than can spot detectors.
Fixed temperature thermal fire detectors rate high on reliability, stability and maintainability but low on sensitivity. In modern buildings with high air flow ventilation and air conditioning systems, placing the fixed temperature detector is a difficult engineering problem. Consequently, this type of thermal fire detector is not widely used outside of very specialized applications.
A rate of rise detector type thermal fire detector is usually installed where a relatively fast-burning fire is expected. The detector operates when the fire plume raises the air temperature within a chamber at a rate above a certain threshold of operation--usually 15.degree. F. per minute. However, if a fire develops very slowly and the rate of temperature rise never exceeds the detector's threshold for operation, the detector may not sense the fire.
The newest type of thermal fire detector is called a rate compensated detector which is sensitive to the rate of temperature rise as well as to a fixed temperature level which is designed into the detector's temperature rating. Even with this dual approach, the most critical problem for effective operation of thermal fire detectors is the proper placement of detectors relative to the hazard area and the occupancy environment. Consequently, this type of fire detector is seldom found in everyday households.
By far the most popular fire detector in use in everyday life today is the smoke detector. Smoke detectors respond to the visible and invisible products of combustion. Visible products of combustion consist primarily of unconsumed carbon and carbon-rich particles; invisible products of combustion consist of solid particles smaller than approximately five (5) microns, various gases, and ions. All smoke detectors can be classified into two basic types: photoelectric type which responds to visible products of combustion and ionization type which responds to both visible and invisible products of combustion.
The photoelectric type is further divided into 1) projected beam and 2) reflected beam. The projected beam type of smoke detectors generally consist of a series of sampling piping from the holds or other protected space on the ship to the photoelectric detector. The air sample is drawn into the piping system by an electric exhaust pump. The photoelectric detector is usually enclosed in a metal tube with the light source mounted at one end and the photoelectric cell at the other end. This type of detector is rather effective due to the length of the light beam. When visible smoke is drawn into the tube, the light intensity of the beam received in the photoelectric cell is reduced because it is obscured by the smoke particles. The reduced level of light intensity causes an unbalanced condition in the electrical circuit to the photocell which activates the alarm. The projected beam or smoke obscuration detector is one of the most established types of smoke detectors. In addition to use on ships, these detectors are commonly used to protect high-value compartments or other storage areas, and to provide smoke detection for plenum areas and air ducts.
The reflected light beam smoke detector has the advantage of a very short light beam length, making it adaptable to incorporation in the spot type smoke detector. The projected beam smoke detector discussed earlier becomes more sensitive as the length of the light beam increases, and often a light beam of 5 or 10 feet long is required. However, the reflected light beam type of photoelectric smoke detector is designed to operate with a light beam only 2 or 3 inches in length. A reflected beam visible light smoke detector consists of a light source, a photoelectric cell mounted at right angles to the light source, and a light catcher mounted opposite to the light source.
Ionization type smoke detectors detect both the visible and invisible particle matter generated by the diffusion flame combustion. As indicated previously, visible particulate matter ranges from 4 to 5 microns in size, although smaller particles can be seen as a haze when present in a high mass density. The ionization detector operates most effectively on particles from 1.0 to 0.01 microns in size. There are two basic types of ionization detectors. The first type has a bipolar ionized sampling chamber which is the area formed between two electrodes. A radioactive alpha particle source is also located in this area. The oxygen and nitrogen molecules of air in the chamber are ionized by alpha particles from the radioactive source. The ionized pairs move towards the electrodes of the opposite sign when electrical voltage is applied, and a minute electrical current flow is established across the sampling chamber. When combustion particles enter the chamber they attach themselves to the ions. Since the combustion particles have a greater mass, the mobility of the ions now decreases, leading to a reduction of electrical current flow across the sampling chamber. This reduction in electrical current flow initiates the detector alarm.
The second type of ionization smoke detector has a unipolar ionized sampling chamber instead of a bipolar one. The only difference between the two types is the location of the area inside the sampling chamber that is exposed to the alpha source. In the case of the bipolar type the entire chamber is exposed leading to both positive and negative ions (hence the name bipolar). In the case of the unipolar type only the immediate area adjacent the positive electrode (anode) is exposed to the alpha source. This results in only one predominant type of ions (negative ions) in the electrical current flow between the electrodes (hence the name unipolar).
Although unipolar and bipolar sampling chambers use different principles of detector design they both operate by the combustion products creating a reduced current flow and thus activating the detector. In general the unipolar design is superior in giving the ionization smoke detectors a greater level of sensitivity and stability, with fewer fluctuations of current flow to cause false signals from variations in temperature, pressure and humidity. Most ionization smoke detectors available commercially today are of the unipolar type.
For the past two decades the ionization smoke detectors have dominated the fire detector market. One of the reasons is that the other two classes of fire detectors, namely the flame sensing detectors and the thermal detectors are appreciably more complex and costlier than the ionization smoke detectors. They are therefore mainly used only in specialized high-value and unique protection areas. In recent years, because of their relatively high cost, even the photoelectric smoke detectors have fallen behind significantly in sales to the ionization type. The ionization type are generally less expensive, easier to use and can operate for a full year with just one 9-volt battery. Today over 90 percent of households that are equipped with fire detectors use the ionization type smoke detectors.
Despite their low cost, relatively maintenance-free operation and wide acceptance by the buying public, the smoke detectors are not without problems and certainly far from being ideal. There are a number of significant drawbacks for the ionization smoke detectors to operate successfully as early warning fire detectors. To state it plainly most people do not complain about them simply because there are no better alternatives.
One of the biggest problems with ionization smoke detectors is their frequent false-alarms. By the nature of its operational principle, any micron-size particulate matters other than the smoke from an actual fire can set off the alarm. Kitchen grease particles generated by a hot stove is one classic example. Over-zealous dusting of objects and/or furniture near the detector is another. Frequent false-alarms are not just a harmless nuisance; some people actually disarm their smoke detectors by temporarily removing the battery in order to escape from such annoying episodes. This latter situation could be outright dangerous especially when these people forget to rearm their smoke detectors.
Another significant drawback for the current ionization smoke detector is its relatively slow speed to alert people of a fire. There are several factors that contribute to this particular drawback. The first fact is the detector trigger threshold for smoke which directly affects its response time to the onset of a fire. No doubt a lower trigger threshold would mean a faster fire detector. However, it also means more frequent annoying false alarms for the user. The second factor is the particular placement of the detector with respect to the spot where fire breaks out. Unlike ordinary gases, smoke is actually a complex sooty molecular cluster that consists mostly of carbon. It is much heavier than air and thus diffuses much slower than the gases we encounter everyday. Therefore, if the detector happens to be at some distance from the location of the fire, it will be a while before enough smoke gets into the sampling chamber of the smoke detector to trigger the alarm. A third factor is the nature or type of fire itself. Although smoke usually accompanies fire, the amount produced can vary significantly depending upon the composition of the material that catches fire. For example oxygenated fuels such as ethyl alcohol and acetone give less smoke than the hydrocarbons from which they are derived. Thus, under free-burning conditions oxygenated fuels such as wood and polymethylmethacrylate give substantially less smoke than hydrocarbon polymers such as polyethylene and polystyrene. As a matter of fact a small number of pure fuels namely carbon monoxide, formaldehyde, metaldehyde, formic acid and methyl alcohol burn with non-luminous flames and do not produce smoke at all.
Yet another drawback of present-day ionization smoke detectors has to do with contaminating our environment. Ionization type smoke detectors use a radioactive matter (Co.sup.60) as the source for alpha particles. Although one can argue that the amount of radioactive material currently found in each ionization smoke detector is very small (probably only tens of milligrams) the number of units in operation however easily runs into tens of millions every year. Thus, the continued usage of this type of smoke detector does pose a serious long-term liability of building up a large amount of unwanted nuclear wastes. Since the half life of Co.sup.60 is well over 1,000 years, the potential danger should not be ignored.
Finally, there are a number of lesser issues one has to deal with when using these low-cost ionization smoke detectors. These include the trouble and cost of having to replace its battery once every year or run the risk of owning a unit that does not work because of lack of power. Also, the presently available ionization smoke detectors are rarely equipped with a visual alarm which is desirable for hearing-impaired persons.
Based upon the discussion presented above, it is clear that a rapid, reliable, low-cost, radioactive-free and maintenance free fire detector would be a most welcome addition to the imperfect world of fire detectors of today.
In U.S. Pat. No. 4,738,266, issued Apr. 19, 1988, Thatcher describes an instrument for sensing a change in the carbon dioxide concentration in ambient air. The device uses an unmodulated broadband infrared source operated at a steady temperature and a single pass band filter. In contrast, in the present invention, the temperature of the source is made to alternate between two temperatures and a dual pass band filter is used. The inclusion of the second pass band in the present invention permits the establishment of a reference signal, which renders the present invention immune to false alarms caused by factors that affect both pass bands equally, such as variations in detector sensitivity due to temperature variations, soot on the mirrors, smoke in the sample, etc. Clearly, these factors are significant for a fire detector, and the ability of the present invention to avoid confounding these factors with the carbon dioxide measurement shows the importance of the differences between the present invention and that of Thatcher.
In U.S. Pat. No. 4,648,396 issued Mar. 10, 1987, Raimer shows apparatus for monitoring the difference in carbon dioxide content between an individual's inspired and expired gas streams. The apparatus is similar to that described in the Thatcher patent, except that a feed-back loop is used to maintain the level of radiation incident on the detector constant regardless of window clouding, temperature-induced component drift, etc. The derivative of the detected signal is checked to rule out noise as a source of error in the carbon dioxide measurement.
A number of other patents show one or more features in common with the present invention, but the patents in this class all make use of moving parts (typically rotating filter wheels) or other components which are deemed to be unsuitable for use in a fire detector. These patents include: U.S. Pat. No. 4 785 184 to Bien et al.; U.S. Pat. No. 4,874,572 to Nelson, et al.; U.S. Pat. No. 4,587,427 to Talbot, et al.
The following patents employ lasers for gas analysis, in contrast to the differential temperature source and dual pass band filter used in the present invention: U.S. Pat. No. 4,535,241 to Eberhardt; U.S. Pat. No. 4,490,043 to Cramp; U.S. Pat. No. 4,489,239 to Grant, et al.; U.S. Pat. No. 3,805,074 to McCormack; U.S. Pat. No. 4,450,356 to Murray, et al.; and U.S. Pat. No. 3,998,557 to Javan.